How to make flickering light?
You may achieve this effect fast using Noise Modifier of the Graph Editor. 


Keyframe the lamp's strenght (in my case 1.000) at the desired frame on a timeline with I. Go to the next frame and keyframe it again (light strenght may be the same, since the modifier will do the job).


Go to Graph Editor, press N to open the properties shelf and add a Noise Modifier. Play with its settings for the different results.